Hitherto, a PS plate having a construction wherein a lipophilic photosensitive resin layer is provided on a hydrophilic support has been widely used as a lithographic printing plate. With respect to the plate making method thereof, in general, a desired printing plate has been obtained by mask exposure (surface exposure) via a lith film and dissolution and removal of a non-image area.
In recent years, the digitalizing technology for electronic processing, accumulating and outputting of image information by using a computer has become widespread, and a variety of new image outputting systems corresponding thereto have been put into practical use. As a result, the computer-to-plate (CTP) technology for scanning light having high directivity such as laser beam according to digitalized image information and directly producing a printing plate without using a lith film is desired, and it is an important technical problem to obtain a printing plate precursor adaptive therewith.
As one of systems for obtaining a lithographic printing plate which can be subjected to scan exposure, a construction wherein a photopolymerization based composition having excellent photosensitive speed is used in an ink-receptive photosensitive resin layer (hereinafter referred to as “photosensitive layer”) to be provided on a hydrophilic support has hitherto been proposed and already marketed. A precursor having such a construction bas desired printing performance such that not only it is simple in development treatment, but also it is excellent in resolution, inking properties, printing resistance, and staining resistance.
The foregoing photopolymerizable composition is basically composed of an ethylenically unsaturated compound, a photopolymerization initiation system, and a binder resin, and the image formation is achieved by the matter that the photoinitiation system absorbs light to generate an active radical and cause addition polymerization of the ethylenically unsaturated compound, thereby insolubilizing the photosensitive layer.
The majority of conventional proposals regarding a photopolymerizable composition capable of being subjected to scan exposure disclose the use of a photoinitiation system excellent in photosensitivity and are described in, for example, Bruce M. Monroe, et al., Chemical Revue, Vol. 93 (1993), pp. 435-448, and R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, Vol. 73 (1993), pp. 81-96.
With respect to a conventional CTP system using a photopolymerizable composition composed of such an initiation system and a visible light source having a long wavelength such as Ar laser (488 nm) and FD-YAG laser (532 nm) as a light source, for the sake of enhancing productivity of the plate making step, it is desired to undergo writing at a higher speed. However, its object has not been achieved yet because an output of the light source is not sufficiently high and sensitivity of the photographic material is not sufficiently high.
On the other hand, in recent years, for example, semiconductor laser which can undergo continuous oscillation in a region of from 350 nm to 450 nm by using an InGaN based material is in the stage of practical use. In a scan exposure system using such a short wavelength light source, since the semiconductor laser can be cheaply produced from the structural standpoint, it has an advantage that while it has a sufficient output, an economical system can be constructed. In addition, it is possible to use a photographic material, the photosensitive region of which is present in a short wave side and which can be worked under a brighter safe light, as compared with the conventional system using FD-YAG or Ar laser.
However, a photoinitiation system having sufficient sensitivity against scan exposure in a short wavelength region of from 350 nm to 450 nm has not been known up to date.
Additionally, what a photoinitiation system having high sensitivity is obtained is a more earnestly desired technology widely in the imaging field (for example, see JP-A-2000-258910, J. P. Faussier, Photoinitiated Polymerization-Theory and Applications: Rapra Review., Vol. 9, Report, Rapra Technology (1998), and M. Tsunooka, et al., Prog. Polym. Sci., 21, 1 (1996)). A photoinitiation system composed of a sensitizing dye and an activator can generate an acid or a base in addition to the foregoing active radical by choosing the activator. For example, such a photoinitiation system is utilized for the image formation such as optical fabrication, holography, and color hard copying, the fabrication field of electronic materials such as photoresists, and photo-curable resin materials such as inks, paints, and adhesives. In these industrial fields, for the sake of causing decomposition of the activator with good efficiency, it is eagerly desired to find out a sensitizing dye having excellent light absorptivity and sensitizing ability.